As described in the above-referenced "536 application, communication service providers are subject to very strict bandwidth usage spectrum constraints, including technically mandated specifications and regulations imposed by the Federal Communications Commission (FCC). These rules require that sideband spillage, namely the amount of energy spillover outside a licensed band of interest, be sharply attenuated (e.g., on the order of 50 dB). Although these regulations may be easily met for traditional forms of modulation, such as FM, they are difficult to achieve using more contemporary, digitally based modulation formats, such as M-ary modulation.
Attenuating the sidebands sufficiently to meet industry or regulatory-based requirements by means of such modulation techniques requires very linear signal processing systems and components. Although relatively linear components can be implemented at a reasonable cost at relatively narrow bandwidths (baseband) of telephone networks, linearizing components such as power amplifiers at RF frequencies can be prohibitively expensive.
A fundamental difficulty in linearizing RF power amplifiers is the fact that they are inherently non-linear devices, and generate unwanted intermodulation distortion products (IMDs) that manifest themselves as spurious signals in the amplified RF output signal, such as spectral regrowth or spreading of a compact spectrum into spectral regions that do not appear in the RF input signal. This distortion causes the phase/amplitude of the amplified output signal to depart from the phase/amplitude of the input signal, and may be considered as an incidental (and undesired) amplifier-sourced modulation of the RF input signal.
An inefficient approach to linearizing an RF power amplifier is to build the amplifier as a large, high power device, and then operate the amplifier at a low power level (namely, at only a small percentage of its rated output power), where the RF amplifier's transfer characteristic is relatively linear. An obvious drawback to this approach is the overkill penalty--a costly and large sized RF device.
Other prior art techniques include baseband polar (or Cartesian) feedback, post-amplification, feed-forward correction, and pre-amplification, pre-distortion correction. In the first approach, the output of the amplifier is compared to the input and a baseband error signal is used to directly modulate the signal which enters the power amplifier. In the second approach, error (distortion) present in the RF amplifier's output signal is extracted, amplified to the proper level, and then reinjected (as a complement of the error signal back) into the output path of the amplifier, so that (ideally) the RF amplifier's distortion is effectively canceled.
In the third approach, a predistortion signal is injected into the RF input signal path upstream of the RF amplifier. The predistortion signal ideally has a characteristic that is equal and opposite to the distortion expected at the output of the high power RF amplifier, so that when subjected to the (distorting) transfer characteristic of the RF amplifier, it effectively cancels the output distortion. Predistortion may be made adaptive by measuring the distortion at the output of the RF amplifier and adjusting the predistortion control signal to minimize the distortion of the output signal of the power amplifier during real time operation.
In accordance with the invention described in the above-referenced '536 application and diagrammatically illustrated in FIG. 1, linearization of a main RF power amplifier A.sub.1 is effectively achieved by using a second RF amplifier A.sub.2, that is largely matched with the main RF power amplifier A.sub.1. Being matched implies that the two RF amplifiers have the same transfer characteristics--both in terms of their intended RF performance and unwanted IMD components they inherently introduce into their amplified outputs.
An RF input signal to be amplified is split by a directional coupler CPL1 into two paths, a first of which adjusts the RF input signal in amplitude and phase prior to being amplified by the amplifier A.sub.1. A second split RF input signal path is used to construct a signal consisting of both of the original RF input signal to be amplified by the second amplifier A.sub.2, and a complementary version of the IMD products the two amplifiers inherently introduce.
By selectively combining the RF output of the RF power amplifier A.sub.1 with the RF input signal in the parallel path, the gain and phase adjusted RF input signal applied to the matched amplifier A.sub.2 can be made to include the same modulated RF carrier component as that applied to power amplifier A.sub.1. In addition, the adjusted RF input signal to the parallel path matched amplifier A.sub.2 will contain a complementary version of the intermodulation component of the output of the main path amplifier A.sub.1.
The gain and phase adjustment of the input to the second amplifier A.sub.2 is such that its RF output signal will have the desired RF carrier modulation component aligned with that of the main path amplifier A.sub.1, but its undesired intermod component will be of equal amplitude and phase-reversed with respect to the undesired intermod component of the RF output of the main path amplifier A.sub.1. Combining these two matched amplifier outputs in a quadrature hybrid CPL2 yields a composite signal, in which the desired amplified modulated RF carrier components produced by each matched amplifier constructively sum to the intended amplification level, while unwanted IMD components destructively combine or cancel, effectively leaving only the desired amplified modulated RF carrier.